Process of curing concrete



Patented Feb. 19, 1935 PATENT OFFICE PROCESS OF CURING CONCRETE Lester Wittenberg,v New Rochelle, N. Y., assignor to The BarrettCompany, New York, N. Y., a. corporation of New Jersey No Drawing. Application August 25, 19

Serial No. 630,427

4 Claims.

This invention is directed to curing concrete, and more particularly, to the curing of concrete by suitably covering the surface of the wet concrete and maintaining the covering on the con- 5 crete until it is uniformly and evenly set.

It is well known that proper curing of concrete is important in order to obtain concrete without'cracks or fissures and which will not become weak and brittle and therefore unsatisfactory for road surfacing, building, and other purposes. To accomplish proper curing of concrete is particularly difficult in the laying of roads and pavements because of the relatively large surface exposed to the air and sun which tend to dry the surface before uniform setting of the concrete has occurred.

It has been proposed to maintain the concrete covered with water by buildingearth dams thereacross at suitable intervals and-filling the intervening spaces with water. This is quite costly; results in an unsightly appearance of the concrete road while under construction; and is not practical when the pavement is laid on a grade. Furthermore, the earth dams have a tendency to impair the concrete. It has been demonstrated that earth of alkaline nature occurring naturally in some localities may seriously impair the strength of the concrete.

Concrete has been covered with straw or pieces of burlap and these materials sprinkled atfrequent intervals with water. Such curing procedure results in an unsightly appearance of the pavement construction and the application of straw and burlap to the concrete requires considerable manual labor. It is also exceedingly difficult to prevent the straw or pieces of burlap from being disturbed by the wind. Burlap or canvas has also beensuspended on frames a short distance above the concrete. Such pro- 'cedure is costly and the frames and covering are difllcult to keep in place in windy weather. It has also been proposed to cover concrete with bitumen saturated roofing felt material that is substantially waterproof. S uch' procedure, 'in addition to requiring the use of the comparatively costly bitumen-saturated felt, renders it difiicult to retain in the top portion of the concrete the necessary water to effect curing since the mixing water in the concrete tends to pass out through the bottom and insufficient water is retained in the top portion of the concrete to provide satisfactory curing.

I have discovered that concrete may be efficiently and economically cured by covering the surface thereof with felted fibrous material, such as unsaturated roofing felt. The felt is preferably applied to the concrete substantially immediately after it is poured and. readily absorbs water from the wet concrete and adheres thereto, no difficulty being experienced in its application or maintenance on the concrete even in windy weather. During the curing of the concrete, which may require as long as fourteen days, or longer, the felt is preferably wetted or sprinkled with water. The wetting may be re- 10 peated at intervals, the frequency depending on the humidity and temperature of the surrounding air, whether salt solution (e. g., calcium chloride) or water alone is used and like factors affecting the water retention of the felt. Two wet- 15 tingswith water per day will be sufficient to insure proper curing except under very dry atmospheric conditions, in which case water may be applied to the felt several times per day. Under very humid conditions, one wetting after appli- 20 cation of the felt may suffice.

Felted fibrous material of open structure and high moisture-absorbing properties, such as made from rags, newsprint, wood pulp, bagasse, or other absorptive vegetable fibers in any de- 25 sired proportions may be employed. It is important that the fibrous material be felted, and for best results, be composed of fibers that are highly water absorptive. Preferably, fibrous material having a porosity of not less than approximately 1.8 cc. of voids per gram is utilized.

To determine the cubic centimeters of voids per gram of felt, a small sample of felt (say 2" x 5") is dried for one hour at 100 C., cooled, desiccated, and thereafter weighed. It is then immersed in kerosene, maintained at a temperature of 25 0., for 18 hours. Thereafter, the sample is removed from the kerosene, drained in vertical position for 30 seconds, and again weighed. The difference in weight represents the kerosene absorbed. This difference in weight in grams, divided by the specific gravity of the kerosene, corresponds to the cubic centimeters of kerosene absorbed. This volume in cubic centimeters, divided by the original weight in grams, gives the voids in cubic centimeters per gram.

In the construction of cement concrete roads, the concrete may be mixed in the proper proportions in a mechanical mixer and deposited on the prepared sub-grade of the highway. The 9 surface of the concrete is then levelled to the desired cross-section ordinarily by means of a mechanical finishing machine of any well-known type. As soon as the concrete has been levelled, dry unsaturated felt, felt paper, or felted material of high moisture absorptive value is applied to completely cover the surface thereof. A felt weighing 28 pounds per 480 square feet and about inch thick will result in satisfactory curing. The felt material absorbs moisture from the concrete and instantly adheres thereto. The thickness of the felt may be varied to suit local climatic conditions, the length of time the covering will be maintained on the concrete, etc. For colder climates preferably thicker felts are em? ployed.

The felt may be made in varying widths best suited to each operation. It is preferably made suificiently wide so that it willcover the. entire width of the concrete poured at one time thus eliminating the necessity of lapping two or more adjacent strips of felt. The felt rolls maybe carried on trucks travelling on the erected side forms usually employed in the pouring of concrete. Such a truck may be moved along continuously following the finishing machine, felt being .unrolled from the truck as it moves and hence immediately protecting the freshly finished surface. If more than one strip of felt is necessary to cover the width of concrete poured, two or more rolls may be mounted in staggered position on the truck and the felt therefrom unrolled and laid in somewhat overlapping parallel lanes.

- The felt may be sprayed with a suitable hygroscopic salt solution, such as a calcium chloride solution, or sea water or the like, or dry calcium chloride or other hygroscopic salt may be placed on top of the felt, preferably after it is applied to the wet concrete. The calcium chloride or other hygroscopic salt will absorb water from the air and may eliminate necessity for sprinkling the felt with water. Approximately one pound of calcium chloride or other hygroscopic salt may be added per square yard of felt.

In cases where the concrete is surface-treated, as for example with salts such' as sodium silicate, the felt may be applied directly to the treated concrete.

Among the numerous advantages possessed by my invention over prior procedures of curing concrete may be mentioned the following:

(l)v It is more economical to practice, particularly as compared with the use of burlap and saturated felt, and results in efficiently cured concrete. Burlap is comparatively expensive and does not have the water absorptive properties of felt. As is well known, burlap is woven from jute, flax, hemp, or Manila fibers. Since it is a woven material, it is necessarily composed of twisted yarn, which is much less absorbent than the fibrous constituents of roofing felt. In using burlap, excessive amounts of water must be applied to obtain penetration of the water through the burlap to the concrete.

(2) Burlap, when applied to wet concrete, due to the fact that it is materially less absorptive than felt, will not immediately absorb moisture from the wet concrete, and in windy weather will be blown about and difiiculty encountered in applying it. These difficulties are not present in the application of the dry felt of this invention.

(3) The application of dry felt is a cleaner operation than the application of burlap, straw, earth, etc., heretofore used and does not result in an unsightly appearance of the roadway during construction. The dry felt can be removed fromthe road and disposed of at the end-of the curing period much more economically than straw, earth, or similar material. When the concrete is covered with bitumen saturated paper or felt,

there is a possibility that the mix-water in the concrete may flow out through the bottom of the concrete and insufiicient water be retained in the concrete to provide satisfactory curing. The use of the felt of this invention will maintain the water on the surface of the concrete and provide a continuous additional supply of water to insure proper curing of the concrete.

(4) Dry felt is superior in its heat insulating properties to the materials used to protect green concrete heretofore. This makes it possible to lay concrete earlier in the spring and later in the fall in localities where frost is encountered. In any locality the use of-the felt of this invention reduces the suddenness and extent of tempera.-

ture changes to which the concrete is subjected with 1 changes in' weather conditions. When wet concrete is covered with wet felt and freezing weather sets in, I have found that the felt, freezing first,- retains a layer of entrapped air between it and .theconcrete. This layer of air acte'as a heat insulator and protects the concrete for: a considerable time against freezing. If theqcold is so extreme that in spite of-thefelt the concrete freezes, the effect of the freezing temper ture is more uniformly distributed in-the concrete and changes in the concrete occur gradually. As a result, surface scaling ofthe concrete is substantially lessened oreliminated, and upon thawing, the concrete gainsstrength more rapidly than would otherwise be the one. (5) The felt, being light in color, i. e., ofallght grayish hue, reflects heat, and at summer temperatures will maintain the concrete at a lower temperature than would be the case if dark materials, such as bitumen saturated felt-of a blackish color or burlap of adark brown color, were employed. Hence, evaporation of the water maintained-1n contact with the concrete is minimized. J

'(6) The felt, because of its felted fibrous characteristics adheres firmly to the. surface of the concrete, which characteristic not-only prevents the felt from being blown away by the wind as noted hereinbefore,. but a cause the concrete on removal of the feltto have a somewhat rough non-skid surface. The felt cation of calcium chloride directly to wetfconcrete, it has been found, results the concrete. In accordance with this invention, the application. of calcium. chloride to the felt covering over the wetcon'crete avoids scaling of the concrete and'at the sa'metime permits the utilization of the. water absorbing and retaining properties of the salt.

Since certain changes may bemade in carrying out the above process without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative andnot in a limiting sense. I 1

1. The method'of, curing concrete, whieh consists in covering the :surfaceof the wetconerete substantially immediately after, it ispoured with light-colored water-absorbent sheet material composed of felted fibers, and maintaining said sheet material moist and in direct communication with the atmosphere during the curing.

2. The method of curing concrete, which consists in covering the surface of the wet concrete substantially immediately after it is poured with light-colored unsaturated roofing felt having not less than 1.8 c. c. of voids per gram of felt and maintaining said felt moist and in direct communication with the atmosphere during the curing.

the wet concrete with light-colored water-ab- 10 sorbent felt saturated with calcium chloride. LESTER WI'I'I'ENBERG. 

